First, an AMC mode which is applied to a low-speed user equipment (UE) in the IEEE (Institute of Electrical and Electronics Engineers) 802.16e system will be described in brief.
FIG. 1 illustrates a structure of one subchannel when an AMC mode is used in the IEEE 802.16e system.
Referring to FIG. 1, a horizontal axis indicates time and a vertical axis indicates frequency. Orthogonal frequency division multiplexing (OFDM) symbols and subcarriers are mapped in a time domain and a frequency domain, respectively, thereby constituting a frame.
In the IEEE 802.16e system, a basic unit structure, called a bin, is used when a specific transmitting side transmits signals in an AMC mode. One bin is comprised of 9 subcarriers including 8 data subcarriers and one pilot subcarrier, as shown in FIG. 1. It is assumed that the term ‘data subcarrier’ refers to a data transmission unit corresponding to one subcarrier in the frequency domain within one OFDM symbol in the time domain, and the term ‘pilot subcarrier’ refers to a pilot transmission unit corresponding to the above-described area.
Meanwhile, a subchannel, which is the smallest basic unit of data transmission in the IEEE 802.16e system, is comprised of 6 bins. The number of data subcarriers constituting the subchannel is 48 (=6×8). Although FIG. 1 illustrates an example in which the subchannel structure has 2 bins in the frequency domain and 3 OFDM symbols in the time domain (i.e., 2 bins×3 OFDM symbols), the bins constituting the subchannel may be variously constructed. For example, the subchannel comprised of 6 bins may be constructed in the form of 1 bin×6 OFDM symbols, 2 bins×3 OFDM symbols, 3 bins×2 OFDM symbols, and 6 bins×1 OFDM symbol.
For transmission of data and pilots, a transmitting side in a general multicarrier system including the IEEE 802.16e system appropriately allocates transmission signals to subcarriers and performs inverse fast Fourier transform (IFFT) for the transmission signals to convert and map the transmission signals to time domain signals. A process of allocating the transmission signals to subcarriers will now be described in brief.
The following Table 1 illustrates orthogonal frequency division multiple access (OFDMA) system parameters in case of a fast Fourier transform (FFT) size of 2048 in the IEEE 802.16e system.
TABLE 1ParameterValueNumber of DC Subcarriers1 (index 1024,count from 0)Number of Guard Subcarriers, left160Number of Guard Subcarriers, right159Nused, Number of Used Subcarriers (which1729includes the DC subcarrier))Total Number of Subcarriers2048Number of Pilots192Number of Data Subcarriers1536Number of Physical Bands48Number of Bins per Physical Band4Number of Data Subcarriers per slot48
In Table 1, a total of 2048 subcarriers include 160 left guard subcarriers, 159 right guard subcarriers, and 1729 used subcarriers. The 1729 used subcarriers include one DC subcarrier, 1536 data subcarriers, and 192 pilot subcarriers.
In addition to an FFT size of 2048 as shown in Table 1, the FFT size may be 1024, 512, 128, etc. The OFDM system parameters when the FFT size is 1024, 512, and 128 are illustrated in Table 2, Table 3, and Table 4, respectively.
TABLE 2ParameterValueNoteNumber of DC Subcarriers1—Number of Guard80—Subcarriers, leftNumber of Guard79—Subcarriers, rightNumber of Used865—Subcarriers (Nused)(including all possibleallocated pilots andthe DC subcarrier)Number of Pilot96—SubcarriersPilot Subcarrier Index9k + 3m + 1,Symbol of index 0 in pilotfor k = 0,subcarrier index should be1, . . . , 95,the first symbol of theand m =current zone. m is[symbolincremented only for dataindex]symbols, excluding pre-mod 3ambles, safety zones,sounding symbols, mid-ambles, etc.DC subcarrier is excludedwhen the pilot subcarrierindex is calculated by theequationNumber of Data Subcarriers768—Number of Physical Bands24—Number of Bins per Physical4—BandNumber of Data Subcarriers48—per slot
TABLE 3ParameterValueNoteNumber of DC Subcarriers1—Number of Guard40—Subcarriers, leftNumber of Guard39—Subcarriers, rightNumber of Used433—Subcarriers (Nused)(including all possibleallocated pilots andthe DC subcarrier)Number of Pilot48—SubcarriersPilot Subcarrier Index9k + 3m + 1,Symbol of index 0 in pilotfor k = 0,subcarrier index should be1, . . . , 47,the first symbol of theand m =current zone. m is[symbolincremented only for dataindex]symbols, excluding pre-mod 3ambles, safety zones,sounding symbols, mid-ambles, etc.DC subcarrier is excludedwhen the pilot subcarrierindex is calculated by theequationNumber of Data Subcarriers384—Number of Physical Bands12—Number of Bins per Physical4—BandNumber of Data Subcarriers48—per slot
TABLE 4ParameterValueNoteNumber of DC Subcarriers1—Number of Guard10—Subcarriers, leftNumber of Guard9—Subcarriers, rightNumber of Used109—Subcarriers (Nused)(including all possibleallocated pilots andthe DC subcarrier)Number of Pilot12—SubcarriersPilot Subcarrier Index9k + 3m + 1,Symbol of index 0 in pilotfor k = 0,subcarrier index should be1, . . . , 11,the first symbol of theand m =current zone. m is[symbolincremented only for dataindex]symbols, excluding pre-mod 3ambles, safety zones,sounding symbols, mid-ambles, etc.DC subcarrier is excludedwhen the pilot subcarrierindex is calculated by theequationNumber of Data Subcarriers96—Number of Physical Bands3—Number of Bins per Physical4—BandNumber of Data Subcarriers48—per slot
As described above, one subchannel in the IEEE 802.16e system is comprised of 6 bins each consisting of 9 subcarriers. One pilot is present per bin with respect to each OFDM symbol in the time domain and is used for channel estimation. 9 subcarriers in the frequency domain are subject to similar channels and each OFDM symbol in the time domain is subject to similar channels. Such a subchannel is the smallest basic unit of signal transmission and it is assumed that one subchannel (or plural subchannels) assigned to the same UE is subject to similar channel environments for transmission during operation in an AMC mode.
The AMC mode during data transmission is applied to a low-speed UE, as compared with a partial usage of subchannels (PUSC) mode applied to a high-speed UE during data transmission. Accordingly, when a specific UE transmits signals in the AMC mode, insertion of pilots into each OFDM symbol does not lead to a significant increase in performance compared to when no pilots are inserted into each OFDM symbol. If pilots are inserted into each OFDM symbol in the AMC mode, since data subcarriers corresponding to the inserted pilots can not be used, an overhead occurs in terms of data transmission. Therefore, an efficient structure can be obtained by reducing the number of pilots and increasing the number of used data subcarriers while maintaining the performance of the system. However, the above IEEE 802.16e system does not propose a solution to such a problem.